U.S. patent number 8,576,519 [Application Number 13/649,245] was granted by the patent office on 2013-11-05 for current-perpendicular-to-the-plane (cpp) magnetoresistive (mr) sensor with magnetic damping material at the sensor edges.
This patent grant is currently assigned to HGST Netherlands B.V.. The grantee listed for this patent is HGST Netherlands B.V.. Invention is credited to Matthew J. Carey, Jeffrey R. Childress, Young-suk Choi, John Creighton Read.
United States Patent |
8,576,519 |
Carey , et al. |
November 5, 2013 |
**Please see images for:
( Certificate of Correction ) ** |
Current-perpendicular-to-the-plane (CPP) magnetoresistive (MR)
sensor with magnetic damping material at the sensor edges
Abstract
A current-perpendicular-to-the-plane magnetoresistive sensor has
magnetic damping material located adjacent either or both of the
sensor side edges and back edge to reduce the effect of spin
transfer torque. The damping material may be Pt, Pd, Os, or a rare
earth metal from the 15 lanthanoid elements. The damping material
may be an ultrathin layer in contact with the sensor edges. An
insulating layer is deposited on the damping layer and isolates the
sensor's ferromagnetic biasing layer from the damping layer.
Instead of being a separate layer, the damping material may be
formed adjacent the sensor edges by being incorporated into the
material of the insulating layer.
Inventors: |
Carey; Matthew J. (San Jose,
CA), Childress; Jeffrey R. (San Jose, CA), Choi;
Young-suk (Los Gatos, CA), Read; John Creighton (San
Jose, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
HGST Netherlands B.V. |
Amsterdam |
N/A |
NL |
|
|
Assignee: |
HGST Netherlands B.V.
(Amsterdam, NL)
|
Family
ID: |
49487877 |
Appl.
No.: |
13/649,245 |
Filed: |
October 11, 2012 |
Current U.S.
Class: |
360/324;
360/324.2 |
Current CPC
Class: |
G11B
5/3932 (20130101); H01L 43/08 (20130101); G01R
33/098 (20130101); H01F 10/329 (20130101); G01R
33/093 (20130101); G11B 5/398 (20130101); G11B
5/3929 (20130101); H01F 10/3272 (20130101); H01F
10/123 (20130101) |
Current International
Class: |
G11B
5/39 (20060101) |
Field of
Search: |
;360/313-327.33
;365/158,171,173 ;324/207.21,252 ;428/811-811.5,611 ;148/108
;257/295,296,414,421 ;438/3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Klimowicz; Will J
Attorney, Agent or Firm: Berthold; Thomas R.
Claims
What is claimed is:
1. A magnetoresistive sensor structure for sensing magnetically
recorded data from a magnetic recording medium, the structure
comprising: a substrate; a first shield layer of magnetically
permeable material on the substrate; a magnetoresistive sensor
comprising a stack of layers on the first shield layer and having a
front edge for facing a magnetic recording medium, a back edge
recessed from the front edge, and two spaced-apart side edges that
define a sensor track width (TW) at the front edge, the sensor
being capable of sensing magnetically recorded data when a sense
current is applied perpendicular to the planes of the layers in the
sensor stack; and a damping layer on and in contact with the sensor
side edges, the damping layer being formed of material selected
from the group consisting of Pt, Pd, Os, La, Ce, Pr, Nd, Pm, Sm,
Eu, Gd, Tb, Dy, Ho, Er, Th, Yb, and Lu; and an insulating layer on
and in contact with the damping layer and on regions of the first
shield layer adjacent the sensor side edges.
2. The sensor structure of claim 1 wherein the damping layer is
also on and in contact with the first shield layer adjacent the
sensor side edges.
3. The sensor structure of claim 1 wherein the thickness of the
damping layer is less than 10 .ANG..
4. The sensor structure of claim 1 wherein the damping layer is a
discontinuous film.
5. The sensor structure of claim 1 wherein the damping layer on and
in contact with the side edges of the sensor is a first damping
layer, and further comprising a second damping layer comprising a
material selected from the group consisting of Pt, Pd, Os, La, Ce,
Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Th, Yb, and Lu on and in
contact with the back edge of the sensor.
6. The sensor structure of claim 5 wherein the second damping layer
is also on and in contact with the first shield layer adjacent the
sensor back edge.
7. The sensor structure of claim 1 further comprising a
ferromagnetic biasing layer on the insulating layer; and a second
shield layer formed of magnetically permeable material on the
sensor and ferromagnetic biasing layer.
8. The sensor structure of claim 7 wherein the ferromagnetic
biasing layer is a layer of hard magnetic material comprising a
CoPt alloy.
9. The sensor structure of claim 1 wherein the damping material
consists essentially of an element selected from Pt, Pd, Gd, Dy, Tb
and Ho.
10. The sensor structure of claim 1 wherein the sensor is a
current-perpendicular-to-the-plane giant magnetoresistance
(CPP-GMR) sensor.
11. The sensor structure of claim 1 wherein the sensor is a
current-perpendicular-to-the-plane tunneling magnetoresistance
(CPP-TMR) sensor having a resistance-area (RA) product less than
0.3 Ohm-.mu.m.sup.2.
12. A current-perpendicular-to-the-plane (CPP) giant
magnetoresistive (GMR) read head structure for reading magnetically
recorded data from tracks on a magnetic recording disk in a disk
drive, the read head structure comprising: an air-bearing slider
having an air-bearing surface (ABS) for facing the disk and a
trailing surface generally orthogonal to the ABS; a first shield
layer of magnetically permeable material on the slider's trailing
surface; a GMR read head comprising a stack of layers on the first
shield layer and having a front edge substantially at the ABS, a
back edge recessed from the front edge, and two spaced-apart side
edges that define a read head trackwidth (TW); a damping layer on
and in contact with at least one of (a) the side edges of the read
head and (b) the back edge of the read head, the damping layer
consisting essentially of an element selected from the group
consisting of Pt, Pd, Os, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,
Ho, Er, Th, Yb, and Lu; an insulating layer on and in contact with
the damping layer and on regions of the first shield layer adjacent
the read head side edges; a ferromagnetic biasing layer on the
insulating layer; and a second shield layer of magnetically
permeable material on the read head and the ferromagnetic biasing
layer.
13. The read head structure of claim 12 wherein the damping layer
consists essentially of an element selected from Pt, Pd, Gd, Dy, Tb
and Ho.
14. The read head structure of claim 12 wherein the thickness of
the damping layer is less than 101.
15. The read head structure of claim 14 wherein the damping layer
is a discontinuous film.
16. The read head structure of claim 12 wherein the damping layer
is on and in contact with both (a) the side edges of the read head
and (b) the back edge of the read head.
17. A magnetoresistive sensor structure for sensing magnetically
recorded data from a magnetic recording medium, the structure
comprising: a substrate; a first shield layer of magnetically
permeable material on the substrate; a magnetoresistive sensor
comprising a stack of layers on the first shield layer and having a
front edge for facing a magnetic recording medium, a back edge
recessed from the front edge, and two spaced-apart side edges that
define a sensor track width (TW) at the front edge, the sensor
being capable of sensing magnetically recorded data when a sense
current is applied perpendicular to the planes of the layers in the
sensor stack; and an insulating layer on and in contact with the
sensor side edges and on regions of the first shield layer adjacent
the sensor side edges, wherein damping material is incorporated
into the insulating layer, said damping material being selected
from the group consisting of Pt, Pd, Os, La, Ce, Pr, Nd, Pm, Sm,
Eu, Gd, Tb, Dy, Ho, Er, Th, Yb, and Lu.
18. The sensor structure of claim 17 wherein the damping material
is a dopant in the material of the insulating layer in an amount
less than 20 atomic percent.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to a
current-perpendicular-to-the-plane (CPP) giant magnetoresistive
(GMR) sensor that operates with the sense current directed
perpendicularly to the planes of the layers making up the sensor
stack, and more particularly to a CPP-GMR sensor with magnetic
damping to suppress spin transfer torque (STT).
2. Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the
read head in magnetic recording disk drives is a "spin-valve"
sensor based on the giant magnetoresistance (GMR) effect. A GMR
spin-valve sensor has a stack of layers that includes two
ferromagnetic layers separated by a nonmagnetic electrically
conductive spacer layer, which is typically copper (Cu) or silver
(Ag). One ferromagnetic layer adjacent the spacer layer has its
magnetization direction fixed, such as by being pinned by exchange
coupling with an adjacent antiferromagnetic layer, and is referred
to as the reference layer. The other ferromagnetic layer adjacent
the spacer layer has its magnetization direction free to rotate in
the presence of an external magnetic field and is referred to as
the free layer. With a sense current applied to the sensor, the
rotation of the free-layer magnetization relative to the
reference-layer magnetization due to the presence of an external
magnetic field is detectable as a change in electrical resistance.
If the sense current is directed perpendicularly through the planes
of the layers in the sensor stack, the sensor is referred to as a
current-perpendicular-to-the-plane (CPP) sensor.
CPP-GMR sensors are susceptible to current-induced noise and
instability. The spin-polarized bias or sense current flows
perpendicularly through the ferromagnetic layers and produces a
spin transfer torque (STT) on the local magnetization. This can
produce magnetic instabilities and even continuous gyrations of the
magnetization in the ferromagnetic layers, resulting in substantial
low-frequency magnetic noise in the measured electrical resistance
if the bias current is above a certain level. This effect is
described by J.-G. Zhu et al., "Spin transfer induced noise in CPP
read heads," IEEE Transactions on Magnetics, Vol. 40, January 2004,
pp. 182-188. To maximize the signal and signal-to-noise ratio (SNR)
in CPP-GMR sensors, it is desirable to operate the sensors at a
high bias current density. However, the adverse effect of STT
limits the bias current at which the sensors can operate. Both the
free layer and reference layers in the sensor are susceptible to
STT, and therefore the layer with the highest sensitivity to STT
will typically limit the performance of the sensor. One proposal to
alleviate this problem to some degree is to increase the magnetic
damping of the ferromagnetic layers, i.e., to increase the
effective thermal coupling between the magnetization (spin-system)
and that of its host lattice. With sufficient damping, the magnetic
layer with magnetization excitations caused by STT will lose energy
to the lattice faster than it can absorb energy from the bias
current via STT.
U.S. Pat. No. 7,423,850 B2, assigned to the same assignee as this
application, describes a CPP-GMR sensor with an antiparallel free
layer (AP-free) structure, i.e., two free layers with
magnetizations oriented antiparallel across a Ru spacer layer,
wherein one of the free layers includes a NiFeTb film for magnetic
damping of the other free layer across the Ru spacer layer. U.S.
Pat. No. 8,233,247 B2, assigned to the same assignee as this
application, describes a scissoring-type CPP-GMR sensor wherein
each of the two free layers is in contact with a magnetic damping
layer formed or Pt, Pd or a lanthanoid.
However, among the most vulnerable parts of the sensor to STT are
the magnetic layer edges where canted or loose spins may be more
readily excited due to their non-collinear orientation with either
the free layer or the pinned layer. What is needed is a CPP-GMR
sensor with increased magnetic damping at the sensor edges to
suppress STT at the most sensitive areas of the sensor without
reducing the sensor signal near the center of the sensor.
SUMMARY OF THE INVENTION
The invention relates to CPP sensors with magnetic damping material
to reduce the effect of spin transfer torque (STT). Magnetic
damping material is located adjacent either or both of the sensor
side edges and back edge. The damping material may be platinum
(Pt), palladium (Pd), osmium (Os), or a rare earth metal from the
15 lanthanoid (formerly called "lanthanide") elements. The damping
material may be an ultrathin layer in contact with the sensor
edges. An insulating layer is deposited on the damping layer and
isolates the sensor's ferromagnetic biasing layer from the damping
layer. The damping layer is ultrathin to not cause significant
electrical shunting or signal degradation. Instead of being a
separate layer, the damping material may be formed adjacent the
sensor edges by being incorporated into the material of the
insulating layer. For example, the material of the insulating layer
may be doped with the damping material in an amount less than 20
atomic percent. If the damping material is incorporated into the
insulating layer, rather than being a layer in contact with the
sensor edges, it will not form an electrical shunting path, which
eliminates the concern of making the separate damping layer
ultrathin.
For a fuller understanding of the nature and advantages of the
present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic top view of a conventional magnetic recording
hard disk drive with the cover removed.
FIG. 2 is an enlarged end view of the slider and a section of the
disk taken in the direction 2-2 in FIG. 1.
FIG. 3 is a view in the direction 3-3 of FIG. 2 and shows the ends
of the read/write head as viewed from the disk.
FIG. 4A is a cross-sectional schematic view of a CPP-GMR read head
structure according to the invention.
FIG. 4B is a sectional view of the CPP-GMR read head structure
according to the invention taken through a plane orthogonal to both
the ABS and to the planes of the layers in the sensor stack.
FIG. 5 is a line drawing representing an actual sensor as seen from
the ABS for explaining the process steps for making the sensor of
this invention.
DETAILED DESCRIPTION OF THE INVENTION
The CPP giant magnetoresistive (GMR) sensor of this invention has
application for use in a magnetic recording disk drive, the
operation of which will be briefly described with reference to
FIGS. 1-3. FIG. 1 is a block diagram of a conventional magnetic
recording hard disk drive. The disk drive includes a magnetic
recording disk 12 and a rotary voice coil motor (VCM) actuator 14
supported on a disk drive housing or base 16. The disk 12 has a
center of rotation 13 and is rotated in direction 15 by a spindle
motor (not shown) mounted to base 16. The actuator 14 pivots about
axis 17 and includes a rigid actuator arm 18. A generally flexible
suspension 20 includes a flexure element 23 and is attached to the
end of arm 18. A head carrier or air-bearing slider 22 is attached
to the flexure 23. A magnetic recording read/write head 24 is
formed on the trailing surface 25 of slider 22. The flexure 23 and
suspension 20 enable the slider to "pitch" and "roll" on an
air-bearing generated by the rotating disk 12. Typically, there are
multiple disks stacked on a hub that is rotated by the spindle
motor, with a separate slider and read/write head associated with
each disk surface.
FIG. 2 is an enlarged end view of the slider 22 and a section of
the disk 12 taken in the direction 2-2 in FIG. 1. The slider 22 is
attached to flexure 23 and has an air-bearing surface (ABS) 27
facing the disk 12 and a trailing surface 25 generally
perpendicular to the ABS. The ABS 27 causes the airflow from the
rotating disk 12 to generate a bearing of air that supports the
slider 22 in very close proximity to or near contact with the
surface of disk 12. The read/write head 24 is formed on the
trailing surface 25 and is connected to the disk drive read/write
electronics by electrical connection to terminal pads 29 on the
trailing surface 25. As shown in the sectional view of FIG. 2, the
disk 12 is a patterned-media disk with discrete data tracks 50
spaced-apart in the cross-track direction, one of which is shown as
being aligned with read/write head 24. The discrete data tracks 50
have a track width TW in the cross-track direction and may be
formed of continuous magnetizable material in the circumferential
direction, in which case the patterned-media disk 12 is referred to
as a discrete-track-media (DTM) disk. Alternatively, the data
tracks 50 may contain discrete data islands spaced-apart along the
tracks, in which case the patterned-media disk 12 is referred to as
a bit-patterned-media (BPM) disk. The disk 12 may also be a
conventional continuous-media (CM) disk wherein the recording layer
is not patterned, but is a continuous layer of recording material.
In a CM disk the concentric data tracks with track width TW are
created when the write head writes on the continuous recording
layer.
FIG. 3 is a view in the direction 3-3 of FIG. 2 and shows the ends
of read/write head 24 as viewed from the disk 12. The read/write
head 24 is a series of thin films deposited and lithographically
patterned on the trailing surface 25 of slider 22. The write head
includes a perpendicular magnetic write pole (WP) and may also
include trailing and/or side shields (not shown). The CPP-GMR
sensor or read head 100 is located between two magnetic shields S1
and S2. The shields S1, S2 are formed of magnetically permeable
material, typically a NiFe alloy, and may also be electrically
conductive so they can function as the electrical leads to the read
head 100. The shields function to shield the read head 100 from
recorded data bits that are neighboring the data bit being read.
Separate electrical leads may also be used, in which case the read
head 100 is formed in contact with layers of electrically
conducting lead material, such as tantalum, gold, or copper, that
are in contact with the shields S1, S2. FIG. 3 is not to scale
because of the difficulty in showing very small dimensions.
Typically each shield S1, S2 is several microns thick in the
along-the-track direction, as compared to the total thickness of
the read head 100 in the along-the-track direction, which may be in
the range of 20 to 40 nm.
FIG. 4A is view of the ABS showing the layers making up a CPP-GMR
sensor structure as would be viewed from the disk. FIG. 4A will be
used to describe the prior art sensor structure as well as the
sensor structure according to this invention. Sensor 100 is a
CPP-GMR read head comprising a stack of layers formed between the
two magnetic shield layers S1, S2. The sensor 100 has a front edge
at the ABS and spaced-apart side edges 102, 104 that define the
track width (TW). The shields S1, S2 are formed of electrically
conductive material and thus may also function as electrical leads
for the sense current I.sub.S, which is directed generally
perpendicularly through the layers in the sensor stack.
Alternatively, separate electrical lead layers may be formed
between the shields S1, S2 and the sensor stack. The lower shield
S1 is typically polished by chemical-mechanical polishing (CMP) to
provide a smooth substrate for the growth of the sensor stack. A
seed layer 101, such as a thin Ru/NiFe bilayer, is deposited,
typically by sputtering, below S2 to facilitate the electroplating
of the relatively thick S2.
The sensor 100 layers include a reference ferromagnetic layer 120
having a fixed magnetic moment or magnetization direction 121
oriented transversely (into the page), a free ferromagnetic layer
110 having a magnetic moment or magnetization direction 111 that
can rotate in the plane of layer 110 in response to transverse
external magnetic fields from the disk 12, and a nonmagnetic spacer
layer 130 between the reference layer 120 and free layer 110. The
nonmagnetic spacer layer 130 is be formed of an electrically
conducting material, typically a metal like Cu, Au or Ag or their
alloys.
The pinned ferromagnetic layer in a CPP-GMR sensor may be a
"simple-pinned" layer or the well-known antiparallel (AP) pinned
structure like that shown in FIG. 4A. An AP-pinned structure has
first (AP1) and second (AP2) ferromagnetic layers separated by a
nonmagnetic antiparallel coupling (APC) layer with the
magnetization directions of the two AP-pinned ferromagnetic layers
oriented substantially antiparallel. The AP2 layer, which is in
contact with the nonmagnetic APC layer on one side and the sensor's
electrically nonmagnetic spacer layer on the other side, is
typically referred to as the reference layer. The AP1 layer, which
is typically in contact with an antiferromagnetic or hard magnet
pinning layer on one side and the nonmagnetic APC layer on the
other side, is typically referred to as the pinned layer. In FIG.
4A the AP-pinned structure has reference ferromagnetic layer 120
(AP2) and lower ferromagnetic layer 122 (AP1) that are
antiferromagnetically coupled across AP coupling (APC) layer 123.
The APC layer 123 is typically Ru, Ir, Rh, Cr or alloys thereof.
The AP1 and AP2 layers, as well as the free ferromagnetic layer
110, are typically formed of crystalline CoFe or NiFe alloys, or a
multilayer of these materials, such as a CoFe/NiFe bilayer. The AP1
and AP2 ferromagnetic layers have their respective magnetization
directions 127, 121 oriented antiparallel. The AP1 layer 122 may
have its magnetization direction pinned by being exchange-coupled
to an antiferromagnetic (AF) layer 124 as shown in FIG. 4A. The AF
layer 124 is typically a Mn alloy, e.g., PtMn, NiMn, FeMn, IrMn,
PdMn, PtPdMn or RhMn. Alternatively, the AP-pinned structure may be
"self-pinned" or it may be pinned by a hard magnetic layer such as
Co.sub.100-xPt.sub.x or Co.sub.100-x-yPt.sub.xCr.sub.y (where x is
about between 8 and 30 atomic percent). Instead of being in contact
with a hard magnetic layer, AP1 layer 122 by itself can be
comprised of hard magnetic or high magnetostriction material so
that it is in contact with an underlayer on one side and the
nonmagnetic APC layer 123 on the other side. In a "self pinned"
sensor the AP1 and AP2 layer magnetization directions 127, 121 are
typically set generally perpendicular to the disk surface by
magnetostriction and the residual stress that exists within the
fabricated sensor. It is desirable that the AP1 and AP2 layers have
similar moments. This assures that the net magnetic moment of the
AP-pinned structure is small so that magnetostatic coupling to the
free layer 110 is minimized and the effective pinning field of the
AF layer 124, which is approximately inversely proportional to the
net magnetization of the AP-pinned structure, remains high. In the
case of a hard magnet pinning layer, the hard magnet pinning layer
moment needs to be accounted for when balancing the moments of AP1
and AP2 to minimize magnetostatic coupling to the free layer.
Instead of an AP-pinned structure, the reference layer may itself
be pinned directly, without the use of an a APC layer or a separate
pinned layer. In this case, usually referred to as a
"simple-pinned" structure, reference layer 120 is directly in
contact with an antiferromagnetic layer 124. Alternatively, this
simple-pinned layer may be comprised of a hard magnet material, or
in contact with a hard-magnet material.
A seed layer 125 may be located between the lower shield layer S1
and the AP-pinned structure. If AF layer 124 is used, the seed
layer 125 enhances the growth of the AF layer 124. The seed layer
125 is typically one or more layers of NiFeCr, NiFe, Ta, Cu or Ru.
A capping layer 112 is located between the free ferromagnetic layer
110 and the upper shield layer S2. The capping layer 112 provides
corrosion protection and may be a single layer or multiple layers
of different materials, such as Ru, Ta, Ti, or a Ru/Ta/Ru,
Ru/Ti/Ru, or Cu/Ru/Ta trilayer. In addition, the capping layer may
include a specific magnetic damping layer, such as Pt or a
lanthanoid material, to further increase the magnetic damping of
the free layer.
In the presence of an external magnetic field in the range of
interest, i.e., magnetic fields from recorded data on the disk, the
magnetization direction 111 of free layer 110 will rotate while the
magnetization direction 121 of reference layer 120 will remain
fixed and not rotate. Thus when a sense current I.sub.S is applied
from top shield S2 perpendicularly through the sensor stack to
bottom shield S1 (or from S1 to S2), the rotation of the free layer
magnetization 111 due to magnetic fields from the recorded data on
the disk will be detectable as a change in electrical
resistance.
The free layer 110 may also consist of a multilayer known as a
antiparallel (AP)-free layer, where two magnetic layers are coupled
so that their magnetizations are antiparallel. In this case, the
two magnetic layers are of unequal magnetizations, so that the
antiparallel-coupled pair has a non-zero net magnetization. This
net magnetization becomes the effective free layer magnetization
which will respond to magnetic fields from recorded data on the
disk.
A ferromagnetic biasing layer 115, such as a CoPt or CoCrPt hard
magnetic bias layer, is also typically formed outside of the sensor
stack near the side edges 102, 104 of the sensor 100. The biasing
layer 115 is electrically insulated from side edges 102, 104 of
sensor 100 by insulating layer 116. An optional seed layer 114,
such as a Cr alloy like CrMo or CrTi, may be deposited on the
insulating layer 116 to facilitate the growth of the biasing layer
115, particularly if the biasing layer is a CoPt or CoPtCr layer. A
capping layer 118, such as layer of Cr, or a multilayer of Ta/Cr is
deposited on top of the biasing layer 115. The upper layer of
capping layer 118, for example Cr, also serves the purpose as a
chemical-mechanical-polishing (CMP) stop layer during fabrication
of the sensor. The biasing layer 115 has a magnetization 117
generally parallel to the ABS and thus longitudinally biases the
magnetization 111 of the free layer 110. Thus in the absence of an
external magnetic field its magnetization 117 is parallel to the
magnetization 111 of the free layer 110. The ferromagnetic biasing
layer 115 may be a hard magnetic bias layer or a ferromagnetic
layer that is exchange-coupled to an antiferromagnetic layer.
FIG. 4B is a sectional view of the CPP-GMR sensor structure of FIG.
4A taken through a plane orthogonal to both the ABS and to the
planes of the layers in the sensor stack. The sensor 100 is thus
depicted with the front edge 106 at the ABS and back edge 108
recessed from the ABS. The front and back edges 106, 108 define the
stripe height (SH) of the sensor 100.
CPP-GMR sensors are susceptible to current-induced noise and
instability. The bias or sense current is spin-polarized as it
flows perpendicularly through the ferromagnetic layers and produces
a spin transfer torque (STT) on the local magnetization of all the
ferromagnetic layers in the sensor, including the free layer 110
and the reference layer 120, irrespective of current direction. As
the bias current is increased, this can produce magnetic
excitations, large-angle rotation or even continuous gyrations of
the magnetization of the ferromagnetic layers, resulting in
substantial magnetic noise. The edges of the free layer 110, i.e.
the side edges 102, 104 in FIG. 4A and the back edge 108 in FIG.
4B, are the most vulnerable parts of the free layer to STT where
canted or loose spins may be more readily excited. Similarly, the
edges of the reference layer are most susceptible.
Thus in the CPP-GMR sensor of this invention magnetic damping
material is located adjacent either or both of the sensor side
edges 102, 104 and back edge 108. This is shown by damping layer
180 in FIG. 4A and damping layer 182 in FIG. 4B. The damping layers
180, 182 are formed of a material consisting essentially of
platinum (Pt), palladium (Pd), osmium (Os), or a rare earth metal
from the 15 lanthanoid (formerly called "lanthanide") elements. The
lanthanoids are lanthanum (La), cerium (Ce), praseodymium (Pr),
neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu),
gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),
erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Pt
and Pd are heavy elements with strong spin-orbit coupling, as
described by Tserkovnyak et al., "Enhanced Gilbert Damping in Thin
Ferromagnetic Films", Phys Rev Lett, Vol. 88, No. 11, 18 Mar. 2002,
117601. The preferred material for damping layers 180, 182 consists
essentially of an element selected from Pt, Pd, Gd, Dy, Tb and
Ho.
In FIG. 4A, the damping layer 180 is in contact with at least the
side edges 102, 104 of the free layer 110 or the reference layer
120. The damping layer 180 may also be formed on the side edges of
the other sensor 100 layers and on regions of shield S1 adjacent
the side edges 102, 104. The insulating layer 116 and optional seed
layer 114 isolate the ferromagnetic biasing layer 115 from the
damping layer 180. In FIG. 4B, the damping layer 182 is in contact
with at least the back edge 108 of the free layer 110 or the
reference layer 120. The damping layer 182 may also be formed on
the back edges of the other sensor 100 layers and on regions of
shield S1 adjacent the back edge 108. An insulating layer 170 is in
contact with the damping layer 182 at the back edge and at region
of the shield S1 adjacent the back edge 108. Insulating layers 116
and 170 may be formed of single or multiple layers of materials
like silicon-nitride (SiN.sub.x), alumina (Al.sub.2O.sub.3), other
insulating nitrides, or other metal oxides like a tantalum (Ta)
oxide and a magnesium (Mg) oxide.
The damping layers 180, 182 are preferably ultrathin, i.e., a
preferred thickness less than 10 .ANG.. The damping layers may be
continuous or discontinuous films. A discontinuous film may be
considered to have an average thickness. As a discontinuous film,
the damping layer material would only partially cover the free
layer or reference layer edges, with the insulating layers being in
contact with the free layer or reference layer edges not covered by
the damping layer material. If the damping layer is sufficiently
thin or discontinuous it will not cause significant electrical
shunting or signal degradation but will improve the output of the
sensor due to higher magnetic damping. This will result in higher
achievable voltage bias with larger magnetoresistance before
excessive STT induced excitations are observed. This effect will be
more pronounced as the sensor size (TW and SH) is decreased because
this increases the ratio of sensor edge surface area to sensor
volume. Also, if the material selected for the damping layer is
capable of forming an oxide, then there may be also be some oxides
of the damping material formed adjacent the sensor side edges if
the subsequently deposited insulating layers 116 and 170 are formed
of alumina (Al.sub.2O.sub.3) or another oxide insulator.
Instead of being a separate layer, the damping material may be
formed adjacent the sensor edges by being incorporated into the
material of the insulating layer. For example, the insulating
material for layers 116, 170 may be doped with small amounts of
damping material. One example is Al.sub.2O.sub.3 doped with less
than 20 atomic percent Dy. If the damping material is incorporated
into the insulating layer, rather than being a layer in contact
with the sensor edges it will not form an electrical shunting path,
which eliminates the concern of making the separate damping layer
ultrathin.
The various fabrication methods and process steps for CPP-MR
sensors are well-known and not part of this invention. FIG. 5 is a
line drawing representing an actual sensor as seen from the ABS and
will be used to briefly explain the process steps for making the
sensor of this invention. First, all of the layers making up the
sensor 100 stack are deposited as full films on S1. A hard mask
material (not shown), like diamond-like carbon (DLC), is deposited
over capping layer 112. A layer of photoresist (not shown) is then
deposited on the DLC. The photoresist is then lithographically
patterned to define the two side edges 102, 104 of the sensor 100.
An ion milling step removes the layers outside the sensor side
edges down to S1. The side regions are then refilled by deposition
of the damping layer 180, insulating layer 116, optional seed layer
114 for the biasing layer 115, the biasing layer 115, and capping
layer 118. A second DLC layer (not shown) is then deposited in the
side regions over the capping layer 118. The photoresist and
deposited material on top of the photoresist are then removed by
chemical-mechanical-polishing (CMP) assisted lift-off down to the
DLC layers. A reactive ion etching (RIE) step then removes the DLC.
This leaves the capping layer 118 in the side regions and the
capping layer 112 above the sensor stack. This is followed by top
cap deposition of a seed layer 101, like Ru/NiFe, over both the
sensor stack and the side regions, and then electroplating of S2 on
layer 101.
The invention is directly applicable to CPP-GMR sensors because of
the desire to increase damping at the sensor edges to minimize STT.
However, the invention may also be beneficial in certain tunneling
magnetoresistance (TMR) CPP sensors. CPP-TMR sensors are well-known
and have a structure similar to the CPP-GMR sensor shown in FIGS.
4A-4B except that the spacer layer 130 is an insulating tunnel
barrier layer like MgO. While it is actually beneficial to reduce
magnetic damping in the free layer of TMR sensors to reduce noise,
for TMR sensors with very low resistances, e.g., a resistance-area
(RA) product less than 0.3 Ohm-.mu.m.sup.2, it may be desirable to
increase damping at the sensor edges to reduce spin-torque-induced
noise.
While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
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